CVE-2026-45537 in OpenSIPS
Summary
by MITRE • 08/05/2026
OpenSIPS is a Session Initiation Protocol (SIP) server implementation. In versions prior to 3.6.6 and 4.0.0-rc1, the construct_uri() function concatenates multiple URI components (protocol, username, domain, port, params) into a fixed 1024-byte global BSS buffer without any bounds checking. When a routing script calls construct_uri() with an attacker-controlled username, a combined component length exceeding 1024 bytes overflows the buffer, corrupting adjacent global data with attacker-controlled content. The overflow reaches disable_503_translation, a global flag controlling SIP 503 response handling, allowing an attacker to deterministically set the flag via the URI username and alter the server's routing behavior for subsequent messages. Because the same buffer is shared with contact_builder(), the overflow also corrupts that function's data, and without a memory sanitizer the adjacent globals are silently overwritten on every request containing a long username. This issue has been fixed in versions 3.6.6 and 4.0.0-rc1.
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Analysis
by VulDB Data Team • 08/05/2026
The OpenSIPS SIP server implementation contains a critical buffer overflow vulnerability in the construct_uri() function that affects versions prior to 3.6.6 and 4.0.0-rc1. This vulnerability stems from the function's use of a fixed 1024-byte global BSS buffer without any bounds checking mechanisms, creating a classic stack-based buffer overflow condition. The flaw occurs when routing scripts invoke construct_uri() with attacker-controlled username data, allowing malicious actors to craft URI components that exceed the designated buffer capacity and overwrite adjacent memory locations.
The technical exploitation of this vulnerability leverages the predictable nature of global memory layout within the OpenSIPS process. When an attacker constructs a URI with an excessively long username component, the concatenation process overflows the 1024-byte buffer and corrupts adjacent global variables. Specifically, the overflow reaches the disable_503_translation flag which controls how the server handles SIP 503 response codes during message processing. This deterministic corruption allows attackers to manipulate the server's behavior regarding error response handling, potentially enabling them to bypass normal routing controls or redirect traffic in unintended ways.
The impact extends beyond simple flag manipulation as the same vulnerable buffer is also shared with the contact_builder() function, creating cascading effects throughout the application's memory management. Memory sanitizer tools would normally detect such overflows, but without these protections, the adjacent global variables are silently overwritten on every request containing a long username component. This means that each malicious request can progressively corrupt additional global state information, making the vulnerability particularly dangerous for long-running servers where multiple requests compound the memory corruption effects.
This vulnerability aligns with CWE-121 Stack-based Buffer Overflow and represents a significant operational risk for SIP infrastructure deployments. The ATT&CK framework categorizes this as a technique involving code injection and privilege escalation through memory corruption, enabling attackers to manipulate core server functionality. The attack vector is particularly concerning because it requires no special privileges beyond the ability to send specially crafted SIP messages to the server, making it exploitable in scenarios where external parties can interact with the SIP endpoint. Organizations using affected OpenSIPS versions face potential disruption of voice services, unauthorized routing of calls, and possible escalation to full system compromise depending on the server's configuration and network exposure.
The fix implemented in versions 3.6.6 and 4.0.0-rc1 addresses the root cause by introducing proper bounds checking mechanisms within the construct_uri() function. This remediation ensures that URI component concatenation respects buffer boundaries while maintaining the application's core functionality. Security practitioners should prioritize upgrading affected installations to prevent exploitation, as the vulnerability enables persistent manipulation of SIP server behavior without requiring authentication or elevated privileges. The issue demonstrates the critical importance of proper input validation and memory management in telephony infrastructure software where even subtle buffer overflow conditions can result in significant service disruption and security compromise.